In this study, we use first-principles electronic structure calculations to investigate the structural, electronic and magnetic properties of pristine and intrinsic vacancy containing Sr₃SnO inverse perovskite. The thermodynamic stability diagram of Sr₃SnO is computed which provides useful information regarding stable synthesis of this material. The limits of atomic chemical potentials of Sr, Sn and O are obtained to determine the relative stability of the formation of metal-atom and oxygen vacancies in Sr₃SnO. The DFT calculations reveal Sr vacancy to be the most stable form of vacancy defect under O-rich conditions, while O and Sn vacancies are found to have low formation energies under O-poor and Sn-poor conditions, respectively. It is concluded that Sr and O vacancy containing Sr₃SnO is non-magnetic, while Sn vacancy containing Sr₃SnO gives rise to stable ferromagnetism. The electronic properties of pristine and Sn, O, and Sr deficient Sr₃SnO are discussed. The Sn vacancy containing Sr₃SnO, displays stable ferromagnetism which originates from spin-polarization of partially filled Sr dangling bonds with a predominant Sr-4d character. Our results explain the origins of experimentally observed room temperature ferromagnetism in non-stoichiometric Sr₃SnO.